1. Field of the Invention
The present invention relates to an image sensing device and an image forming apparatus using the image forming device and, more particularly, to image sensing and image forming techniques suitable for a color image forming apparatus for obtaining a multicolor image (color image), e.g., an electrophotographic copying machine, a laser beam printer, a color printer, or a printing apparatus having a plurality of image forming units (image forming means).
2. Related Background Art
Conventional image forming apparatuses for forming multicolor images ordinarily performs an image forming process in which images in different colors, corresponding to a multicolor image, are formed by a plurality of image forming units and are transferred, by being superposed one on another, onto a sheet of paper transported by a transport means, e.g., a transport belt.
In such a process, even slight misregistration results in considerably degrading the image quality if a full-color image is formed by performing multicolor development in particular. For example, in the case of 400 dpi, even an amount of misregistration not exceeding a fraction of a pixel size of 63.5 μm, appearing as color registration error or a change in color tone, results in considerably degrading the image quality.
Conventionally, multicolor development is performed by using a single image forming unit, i.e., one scanning lens system. That is, optical scanning with fixed optical characteristics is performed to decrease the amount of image registration. This method, however, entails a problem in which a long time is required to output a multicolor or full-color image.
A solution of this problem is the method of forming images in different colors with separate optical scanning devices and superposing the images on a sheet of paper transported by a transport unit. If this method is used, there is a concern about color misregistration when images are superposed.
Japanese Patent Application Laid-open No. 7-261628 discloses an image forming apparatus capable of obtaining a good color image by correcting color misregistration in such an image superposition process.
In the image forming apparatus disclosed in this application as an apparatus which forms a multicolor image by performing multicolor development to form corresponding images in component colors by and by superposing these images, each of image forming units is controlled on the basis of a signal output from a position sensing means which includes an illumination system having a light emitting diode (LED) light source for illuminating with infrared light a position sensing mark for sensing the position of an image transferred onto an image transfer region, and an optical system for imaging the mark on a photodetector. The optical axis of the optical system is perpendicular to the surface on which the transferred image is formed.
In the process of forming a multicolor image by using this position sensing means to sense the position sensing mark, by controlling each image forming unit for multicolor development, and by superposing corresponding images, a sensing error due to a vertical displacement of a means for conveying images, e.g., a transfer belt can be minimized to enable the mark to be detected with accuracy, so the multicolor image to be obtained with improved quality.
FIG. 13 is a diagram showing an image sensing device for sensing color misregistration caused as described above.
Referring to FIG. 13, a position sensing pattern (image) 86 drawn on a recording member 84, which is an intermediate transfer belt, is irradiated with light emitted from a light emitting portion 81a of a light source means 81, condensed by a condensing portion 81b, and reaching the pattern by passing through a dustproofing glass 87.
Light specularly reflected by the position sensing pattern 86 is sensed by a light receiving means 83 through the dustproofing glass 87, a stop 85 and an imaging lens 82.
Image forming units (image forming means) are controlled on the basis of sensing signals obtained by the light receiving means 83 to output images in component colors.
FIG. 14 shows a state where, in the image sensing device shown in FIG. 13, a light beam emitted from the light source 81 at an emission angle of 6.8 degrees travels to the light receiving means 83.
FIG. 25 is a diagram showing another image sensing device for sensing color misregistration caused as described above.
Referring to FIG. 25, a position sensing pattern (image) 106 drawn on a recording member 108, which is an intermediate transfer belt, is irradiated with light emitted from a light emitting portion 101a of a light source means 101, condensed by a condensing portion 101b and an illumination lens 104, and reaching the pattern by passing through a dustproofing glass 107.
Light specularly reflected by the position sensing pattern 106 is sensed by a light receiving means 103 through the dustproofing glass 107, a stop 105 and an imaging lens 102.
Image forming units (image forming means) are controlled on the basis of sensing signals obtained by the light receiving means 103 to output images in component colors.
FIGS. 18A through 18D are diagrams showing states of color misregistration in an image forming apparatus.
The position of an image in a normal state is indicated by 7 and the position of the image (image area) in a case where color misregistration has occurred is indicated by 8 (8a, 8b, 8c, or 8d). In FIGS. 18A, 18B, and 18C showing cases where color misregistration has occurred in the main scanning direction (the direction along the Y-axis), the two lines are illustrated in a state of being spaced apart from each other in the direction of conveyance (the sub-scanning direction, the direction along the X-axis) for ease of explanation. FIG. 18A shows color misregistration due to an inclination of the main scanning direction, as in a case where there is an inclination between an optical unit and a photosensitive drum. In such a case, misregistration is corrected in the direction of the arrow by, for example, adjusting the position of the optical unit, the photosensitive drum or a lens. FIG. 18B shows color misregistration due to a variation in the width of main scanning lines, as in a case where there is a variation in the distance between the optical unit and the photosensitive drum. Such misregistration is liable to occur if the optical unit is a laser scanner. In such a case, misregistration is corrected in the directions of the arrows by changing the length of scanning lines through fine adjustment of the image frequency (increasing the frequency if the scanning width is longer), for example. FIG. 18C shows a writing position error in the main scanning direction. If the optical unit is a laser scanner, the error is corrected in the direction of the arrows by, for example, adjusting timing of writing from the sensed beam position, for example. FIG. 18D shows a writing position error in the sheet transport direction (the direction along the X-axis). This error is corrected in the direction of the arrow by, for example, adjusting timing of writing in each color from the sensed sheet edge position.